fb > Fcr ??? (Girder Stability)

[CE520]

I am planning on erecting a girder over a steam. The problem I am running into is that when the first girder is at rest (simple span,) fb is greater than Fcr, besides supporting the girder in-between the span is there anything I can do to increase Fcr or reduce fb. (Notes Fcr and fb taken from AISC 9th edition)

The Properties of the girder are as follows
(all properties are in inches unless noted)

TF -0.75x16
WEB -44x0.5
BF -1x20
Depth - 45.75
Length of Beam - 111'
Span - 110'
Weight - 20,900 lbs
W = .188 k/ft
NA (from bottom) - 19.639
Ix = 19074 in^4
Stop - 730.502 in^3

Mmax = wL^2/8 = (0.188*111^2/8)*12 = 3474.52 k*in
fb = Mmax/Stop = 4.76 ksi
Fcr = 4.62 ksi (taken at 110' span)

thus where fb (should be) < Fcr it is not.

Thank you for your time.

 

[Lion06]

Box it. Weld a side plate from top flange to bottom flange to create a closed shaped. This will give you a higher LTB capacity. Though, truth be told, I've never looked at this at 110'.

 

[CE520]

the girder has to remain as is, nothing can be added to or taken away from the girder, though you are correct in boxing it

 

[Lion06]

Have you taken advantage of the Cb factor?

 

[nutte]

Check it using the 13th edition specification. Quite a bit changed in the LTB equations since the 9th edition.

If you can't get the stresses to work, you'll have to support it, with a shoring tower below or a holding crane above.

Also read Fundamentals of Beam Bracing by Yura.

http://www.aisc.org/store/p-994-fundamentals-of-beam-bracing.aspx

 

[PMR06]

Would you gain anything by having the larger flange on top?

 

[nutte]

I doubt the engineer who designed the bridge would want his girder installed upside down.

 

[PMR06]

nutte, I doubt he would want it buckling over either! LOL

My suggestion was more of a thought experiment. I don't have time to run the calc myself.

Honestly, the whole setup sounds dangerous. That long a span unbraced?! The existing beam could already be slightly bent out of shape, etc. What about lateral load from wind or accidental impact?

 

[Ron]

the girder has to remain as is, nothing can be added to or taken away from the girder, though you are correct in boxing it

...sounds like a homework problem.

 

[nutte]

Homework? How many students are using the 9th edition to run their homework problems? This is a common problem faced with the erection of bridge girders. The state DOTs that are building the bridge often don't allow the erector to add material to the girders, or perform any extraneous welding on the girders.

 

[CE520]

well this analysis is for the first girder only, thus before any diaphragms (4 even spaces along the bridge) are installed with the placing of the second girder. The temporary wind load that I came up with is based on a 70 mph wind ~ 1304 k-in.

Also Nutte i have checked it with the 13th edition as well it actually creates a larger difference between the 2.

 

[CE520]

well said nutte, Thank you

 

[MiketheEngineer]

So don't install it on a windy day and get the second in place and braced ASAP!! Done all the time.

 

[grantstructure]

Can you not panelize the first girder with the second and erect them at the same time?

 

[dhengr]

I agree with Grant. I would assemble the first and second girders and the four diaphragms btwn. them on the ground and you would have a stable system when you lifted it into place. The difference is the need to rent a 15-20 ton crane for the single girder lift and a whole bunch of bracing or cribbing to provide stability; or in the ground assembled, 2 girder, case you would need a 30-40 ton crane but much fewer hassles.

 

[bridgebuster]

Weld a short length of plate on top and ask for a change order. WHo says nothing can be added? If you're correct the designer didn't check the constructability requirements of AASHTO LRFD.

Here's another thought: Can you put a temporary support 1 foot in from the centerline of bearings? This way your fb will equal your Fcr.

 

[CE520]

I cannot panelize the 2 girder because I do not have the money in for the larger crane also the radii I am setting the girders at is 80 to 85 ft, this turns into a 175 ton crane (I am setting from behind the abutments) so if I panelized the girders it would turn into about a 300 ton crane. (didn't check capacities just estimating)

Yes bridge buster placing a temporary support slightly in will allow my fb = Fcr. Was checking if there was a solution i may have been over looking

The only other way I have thought of to come up with is to clamp w/ heavy duty C-clamps, a plate over the middle where the max moment happens or clamp a bar joist to top flange but that bar joist ends up being about 70', which equals a lot of clamps.

 

[ToadJones]

yea, I was wondering how big the crane would need to be....that's a helluva reach for a girder weighing 21 kips.
Often people assume an "X" ton crane can lift "X" tons no matter what....obviously not the case.

 

[gwynn]

Sounds like a situation where it may be worth looking at launching or crane assisted launching rather than crane erecting the girders.

Other than that you could look at moving the supports in, as others have suggested, or providing a temporary king post if you have enough clearance.

 

[dhengr]

I agree with Toad, I should have said a single girder lift of 15-20 tons vs. a 2 girder lift of 30-40 tons, meaning about doubling the lift requirements, since I know nothing about the actual girders or of the soil or site conditions, or pick radius, etc. for the crane. But, you have worried at some length about fb vs. Fcr; and the bigger problem is bracing that girder, a great big sail in the wind, against lateral loads until you get two girders tied together. And, when you do that bracing you might have solved your fb problem too. Gwynn’s idea sounds like a good one too, and would not necessarily double the crane size, but might necessitate a different location, and different erection methods.